As mobile devices have been continually developed, and the demand for such mobile devices has increased, the demand for secondary batteries as an energy source for such mobile devices has sharply increased. Among such secondary batteries is a lithium secondary battery having high energy density and high discharge voltage, into which much research has been carried out and which is now commercialized.
Based on the appearance thereof, lithium secondary batteries may be generally classified into a cylindrical battery cell, a prismatic battery cell, and a pouch-shaped battery cell. Based on the type of an electrolyte, lithium secondary batteries may also be classified into a lithium ion battery, a lithium ion polymer battery, and a lithium polymer battery.
The recent trend toward the miniaturization of mobile devices has increased the demand for a prismatic battery or a pouch-shaped battery, which has a small thickness. In particular, much interest is currently focused on a pouch-shaped battery cell because it is easy to modify the shape of the pouch-shaped battery, the cost of manufacturing the pouch-shaped battery is low, and the pouch-shaped battery is lightweight.
In general, a pouch-shaped battery cell is a battery having an electrode assembly and an electrolyte contained in a pouch-shaped battery case, made of a laminate sheet including a resin layer and a metal layer, in a sealed state. The electrode assembly, which is mounted in the battery case, may be configured to have a jelly-roll (wound) type structure, a stacked type structure, or a combination (stacked/folded) type structure.
FIG. 1 is a view typically showing the structure of a battery cell including a stacked type electrode assembly.
Referring to FIG. 1, a battery cell 10 is configured to have a structure in which an electrode assembly 30, including positive electrodes, negative electrodes, and separators disposed respectively between the positive electrodes and the negative electrodes, each of the separators being coated with a solid electrolyte, is mounted in a pouch-shaped battery case 20 in a sealed state such that two electrode leads 40 and 41 electrically connected to positive electrode and negative electrode tabs 31 and 32 of the electrode assembly 30 are exposed outward.
The battery case 20 includes a case body 21 having a depressed receiving part 23, in which the electrode assembly 30 is located, and a cover 22 integrally connected to the case body 21.
The battery case 20 is made of a laminate sheet, which includes an outer resin layer 20a constituting the outermost layer of the laminate sheet, a metal barrier layer 20b for preventing the penetration of materials, and an inner resin layer 20c for sealing.
The positive electrode tabs 31 and the negative electrode tabs 32 of the stacked type electrode assembly 30 are respectively coupled to the electrode leads 40 and 41 by welding. In addition, insulative films 50 may be attached to the top and bottom of each of the electrode leads 40 and 41 in order to prevent the occurrence of a short circuit between a thermal bonding device (not shown) and the electrode leads 40 and 41 and to secure a seal between the electrode leads 40 and 41 and the battery case 20 when the upper end 24 of the case body 21 and the upper end of the cover 22 are thermally bonded to each other using the thermal bonding device.
FIG. 2 typically shows a process of forming a sealed portion of the laminate sheet of FIG. 1 and the coupled section of the laminate sheet.
Referring to FIG. 2 together with FIG. 1, the laminate sheet includes an outer resin layer 20a constituting the outermost layer of the laminate sheet, a metal barrier layer 20b for preventing the penetration of materials, and an inner resin layer 20c for sealing.
The outer resin layer 20a functions to protect the battery cell from the outside. For this reason, it is required for the outer resin layer 20a to exhibit high tensile strength and weather resistance in consideration of the thickness thereof. The outer resin layer 20a is generally made of oriented nylon (ONy). The metal barrier layer 20b functions to prevent air or humidity from being introduced into the battery cell. The metal barrier layer 20b is generally made of aluminum (Al). The inner resin layer 20c of the case body 21 and the inner resin layer 20c of the cover 22 are thermally bonded to each other by the application of heat and pressure thereto in the state in which the electrode assembly is mounted between the case body 21 and the cover 22 in order to seal the battery cell. The inner resin layer 20c is generally made of cast polypropylene (CPP).
In the battery case having the multi-layered laminate structure described above, the inner resin layer 20c of the case body 21 and the inner resin layer 20c of the cover 22 face each other. The inner resin layer 20c of the case body 21 and the inner resin layer 20c of the cover 22 are coupled to each other by thermal bonding. As a result, the end of the coupled inner resin layer 20c is exposed outward. Humidity may easily penetrate into the inner resin layer 20c, which is generally is made of a polymer resin. In addition, an electrolyte may leak from the battery case through the end of the coupled inner resin layer 20c. While the battery cell is used for a long period of time, therefore, the lifespan and stability of the battery cell may be reduced.
Therefore, there is a high necessity for technology that is capable of preventing the penetration of humidity and the leakage of an electrolyte, thereby securing the lifespan and stability of a battery cell.